This study will consist of a blood collection from 10-20 HIV-positive outpatients for use in determining patterns of HIV tropism. Whole blood samples will be shipped immediately after collection to an off-site laboratory for the isolation of HIV virus and subsequent analysis of HIV co-receptor usage and SI/NSI phenotype. Patients will not receive study drug. This study is to be performed in support of a planned phase I/II study with AMD-3100, which inhibits HIV fusion to CD4+ cells by interaction at the CXCR4 co-receptor. Virus isolated from the individual samples will be tested in vitro for its sensitivity to AMD-3100 and results correlated with the pattern of co-receptor use.